Control motors for servo-valves have been known for a long time. Such control motors are used in connection with a servo-valve for changing a small electrical input signal in an analogue manner to a hydraulic output signal. Different types of such control motors are known. The book "The Hydraulic Trainer" (published by Mannesmann Rexroth in 1981) shows one type of such a control motor (at page 49) as comprising a plate-shaped horizontally located armature. The armature is arranged with its two ends between permanent magnetic fields. Two control coils having the same magnetic polarity can energize the spaced apart ends of the armature which is supported by a resilient tube. Due to such energization a tilting movement of the armature can be created. The tilting movement of the armature is transmitted to a rod connected with said armature. The rod, in turn, controls a hydraulic amplifier.
German laid-open application DE-OS 33 38 602 discloses a control motor having an armature in the form of an elongate hollow cylindrical design. Said armature is surrounded within the housing of the control motor by a control coil. A rod connected with said armature acts as an output member and can directly or indirectly act on a hydraulic component. The reciprocal movement of the lower end of the rod occurs in accordance with a control current supplied to the control coil. Depending on the task of the control motor, the control current can have different frequencies, so that the control motor will show a certain frequency dependency. If one assumes that the control coil is supplied with a current of about 700 mA, a maximum or 100% displacement will be reached for the frequency f=0, i. e. the lower end of the output member covers between its two extreme positions, for instance, a distance of 0.8 mm, i.e. 0.4 mm between the center position as well as each one of said extreme positions. With increasing frequency the displacement or amplitude becomes smaller than the 100% value. For a certain frequency an increase of the displacement can occur even to values above 100%. This occurence depends also on the hydraulic system which is actuated by the lower end of the output member. This means, expressed in absolute values: it can occur that the control motor achieves instead of the maximum desired admissible displacement or amplitude value of 0.8 mm, an amplitude of 1.5 mm. This can lead to an excessive load for the components of the control motor, specifically the resilient tube supporting the armature.
German laid-open application DE-OS 34 13 959 discloses a direct driven electro-hydraulic servo-valve which is dampened without providing a speed sensor in the main body of the servo-valve.
German laid-open application DE-OS 35 33 817 discloses a control motor, particularly for a control valve. Frequency-depending passive electric dampening means are provided in the control circuit which comprises the control coil means. This way, mechanical overload situations due to resonance are avoided to a large degree for the components. According to a modification described in said document, a dampening coil is located in the coil body adjacent to the control coil. Said dampening coil can be short-circuited.
Also attention is drawn to the following documents: U.S. Pat. No. 2,162,465, British patent 532,905, U.S. Pat. No. 3,301,141 and German patent 341 525.
It is an object of the present invention to modify the control motor of DE-OS 34 33 817 (corresponding to U.S. Pat. No. 4,761,575) in such a manner that while maintaining at least the same good mechanical and electrical properties the design is simplified, so that the costs of manufacturing are reduced.
It is another object of the invention to provide a control motor which can be more readily assembled.